If the source of a Cobalt unit is replaced by one with twice the activity, what happens to the output factor for a 6 cm x 6 cm field?

The output factor is a ratio that represents the dose delivered to a specific field size relative to a reference field size, taking into account factors such as collimation effects and beam geometry. In the case of a Cobalt unit, the output factor is influenced by the activity of the radiation source, field size, and distance.

When the source activity is increased—such as switching to one with twice the activity—this does not change the fundamental characteristics of the beam’s interaction with the patient or the surrounding environment in terms of output factor calculations at a fixed field size. Specifically, an increase in source activity generally results in a higher dose delivered; however, it does not typically affect the geometry or the relative dose distribution within the confines of the specified field sizes directly.

In this context, increasing the source activity would not lead to an immediate and direct outcome of a zero output factor for a 6 cm x 6 cm field. A zero output factor would imply that no radiation is being delivered to that field size, which is not a typical condition when the source's activity is merely doubled.

Therefore, the correct understanding is that increasing the source activity does not affect the output factor itself for a specific given field size in this manner. Thus, the output